1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit and a liquid crystal display (LCD) device using the same.
2. Description of the Related Art
Presently, LCD devices are being developed as the next generation of display devices because of they are light, thin, and consume less power. In general, an LCD device is a non-emissive display device that displays images using a refractive index difference utilizing optical anisotropy properties of a liquid crystal material that is interposed between two substrates.
Among the various type of LCD devices commonly used, active matrix LCD (AM-LCD) devices have been developed because of their high resolution and superiority in displaying moving images. The AM-LCD device includes a thin film transistor (TFT) in each pixel region as a switching device, a pixel electrode in each pixel region, and a second electrode used for a common electrode. One of the two substrates includes a plurality of gate lines and a plurality of data lines crossing the plurality of gate lines in a matrix to define a plurality of pixel regions. A thin film transistor having a gate electrode connected to the gate line, a source electrode connected to the data line, and a drain electrode spaced apart from the source electrode is positioned in each pixel region. Specifically, the thin film transistor is positioned adjacent to the crossing of a gate line and a data line. To drive the LCD device, the gate lines are sequentially driven and simultaneously the data lines are driven by applying a data voltage to the source electrode of the thin film transistor such that the data voltage is applied when the thin film transistor is in ON state.
FIG. 1 is a schematic of an AM-LCD device according to the related art. As shown in FIG. 1, an LCD panel 2 includes a plurality of gate lines GL1 to GLm (m is a positive fixed number) and a plurality of data lines DL1 to DLn (n is a positive fixed number) crossing the plurality of gate lines GL1 to GLm to define a plurality of pixel regions P1 to Pn. A gate driver 6, a data driver 4 and a gamma voltage generator 8 are disposed outside the LCD panel 2 such that the gate driver 6 is connected to the plurality of gate lines GL1 to GLm, the data driver 4 is connected to the plurality of data lines DL1 to DLn, and the gamma voltage generator 8 is connected to the data driver 4. In the alternative, the data driver 4 may include the gamma voltage generator 8. Although not shown, forming the LCD panel 2 includes forming a thin film transistor on a first substrate and a pixel electrode connected to the thin film transistor, forming a color filter on a second substrate and a common electrode on the color filter, attaching the first and second substrates, and injecting a liquid crystal layer between the first and second substrates.
FIG. 2 is a perspective view of an LCD device according to the related art. As shown in FIG. 2, an LCD panel 2 includes a first substrate 15, a second substrate 17 facing the first substrate 15, and a liquid crystal layer 13 interposed between the first substrate 15 and the second substrate 17. A color filter 21 is formed on the second substrate 17 and a common electrode 19 is formed on the color filter 21. A plurality of thin film transistors T is formed on the first substrate 15. A thin film transistor T is positioned in a pixel region P defined within the display image unit.
A plurality of gate lines 29 and a plurality of data lines 31 crossing the plurality of gate lines 29 are formed on the first substrate 15. Each gate line 29 and each data line 31 are connected to a thin film transistor T. First and second polarizers 23 and 25 are disposed on the first substrate 15 and under the second substrate 17, respectively.
A polarized light from a backlight unit (not shown) under the LCD panel 2 is polarized by polarizer 23 and transmitted to the liquid crystal layer 13. When a voltage is applied to the liquid crystal layer 13, an arrangement of the liquid crystal layer 13 is changed. More particularly, a reflective index of the liquid crystal layer 13 is changed, thereby obtaining a desired image by polarization.
FIG. 3 is a cross-sectional view of an LCD device illustrating polarization of light by a polarizer of the related art LCD device. FIG. 4 is a x-y graph showing a direction of a polarization axis for first and second polarizers of FIG. 3 in the related art LCD device. As shown in FIG. 3, a LCD panel 2 corresponds to a normally black mode in that the LCD panel 2 has a black luminance in an OFF state. A first polarization axis of a first polarizer 25 under a first substrate 17 and a second polarization axis of a second polarizer 23 on a second substrate 15 are perpendicularly arranged at about 90 degrees (88 degrees to 92 degrees) with respect to each other.
A first light A from a backlight (not shown) includes all angles except 0, 90, 180 and 270 degrees and a second light B from the backlight includes angles of 0, 90, 180 and 270 degrees. When the first light A enters into the LCD panel 2, the first light A is not transmitted through the LCD panel by the first and second polarizers 25 and 23 when the LCD panel is in a black state. This is because none of the first light A directions correspond to one of the first polarization axis and the second polarization axis. In other words, the LCD panel 2 does not have a light leakage phenomenon in the black state when the first light A enters the LCD panel 2. Conversely, when the second light B enters the LCD panel 2, the second light B is not absolutely absorbed in the LCD panel 2 and a light leakage phenomenon can occur in the black state.
FIGS. 5A and 5B are illustrations showing a luminance distribution of the related art LCD device of FIG. 3. The luminance distribution for the first and second lights A and B from the backlight unit is generally circular or oval. Much of the second light B is coming from the backlight unit is transmitted through the related art LCD device during an OFF state. Therefore, a black luminance in the related art LCD device is increased and the contrast ratio of the LCD device is decreased.